GAV replacement
- Thread starter bigd1101
- Start date
johnbuckley
Well-known member
Usually straightfoward. Firstly ensure your carb is the type with a brass seat(!) later ones didn't have the seat insert. Remove the brass housing for that takes the GAV needle and peer down the into the depths - I find that bright sunlight is as good as anything for peering into the depths. Have a decent screwdriver ( use the slot on the comp jet as a guide to what a good fitting screwdriver is !) you may need to grind the screwdriver to ensure it is a snug fit on the slot and that it fits down into where the seat is without damaging the housing threads. If the carb has been in recent use then it usually unscrews without much fuss . If the carb has been sitting around for years in a damp garage then soaking it in cleaner and/or apply some blowlamp heat may be needed.
1. For brass seat if it has one...
Use a screw driver which fits the slot correctly.
If it will not unscrew STOP before the slot is damaged.
Use a MAPP gas torch (MAPP is hotter than Propane) to heat the area below the brass plug, where brass seat lives. Let it cool. Dunking in cold water will sometime crack casting.
2. If brass GAV housing is stuck heat outside of lower carb casting and brass housing with MAPP torch. Brass will melt long before the cast iron does. (There are two wrench sizes 13/32nds and 7/16ths.)
Oxy-acetylene will melt brass easily use caution.
Use a screw driver which fits the slot correctly.
If it will not unscrew STOP before the slot is damaged.
Use a MAPP gas torch (MAPP is hotter than Propane) to heat the area below the brass plug, where brass seat lives. Let it cool. Dunking in cold water will sometime crack casting.
2. If brass GAV housing is stuck heat outside of lower carb casting and brass housing with MAPP torch. Brass will melt long before the cast iron does. (There are two wrench sizes 13/32nds and 7/16ths.)
Oxy-acetylene will melt brass easily use caution.
Last edited:
bigd1101
Senior Member
It's a 1928 A so I'll assume the carb has a removable screw in seat. I'll look this weekend. I took the old GAV off and tried cleaning it but it still binds a bit when you try to adjust it so I'm replacing the whole unit, cheap fix. I just figured why not replace the old brass seat too, while I'm at it. I replaced the fire wall grommet, the GAV rod grommet, and cleaned up the tube that the rod runs through but the old GAV housing and needle valve just likes to turn hard even though I cleaned it with fine steel wool and oiled it a bit with WD-40. The whole unit was froze up when I got the car. I just want to make sure when I adjust the dash knob I'm getting the right mix and a good seat.
Big hammer
Well-known member
I’ve taken apart serval carbs first I soak in Evaporust and they come apart pretty easily. The trick is a good fitting screwdriver! why the Evaporust works I’am not sure but the screws and seats come out with no heat !
29er
Senior Member
Tom Endy
Well-known member
The first thing you want to do is determine if the GAV seat is cast or a screw in brass seat. Do that by shining a flashlight down into the tunnel and look for a screwdriver slot. if the seat is brass and it does not unscrew easily do not force it as it is very easy to damage the screwdriver slot and make it more difficult to remove. In this case use a torch to heat the casting in the outside area of the brass seat. Get the casting hot enough to observe a slight red glow. Then immediately dunk the entire casting into a bucket of cold water. This usually shocks it loose and it is then easy to unscrew it.
It is prudent to use this technique on any brass part on a Zenith that is frozen in place.
Also be careful not to confuse the brass GAV seat with the comp jet, they look almost identical, but they are not.
Tom Endy
It is prudent to use this technique on any brass part on a Zenith that is frozen in place.
Also be careful not to confuse the brass GAV seat with the comp jet, they look almost identical, but they are not.
Tom Endy
Tom Wesenberg
Well-known member
X2 what Tom just said. I've done it that way for years, and never cracked any parts.
bigd1101
Senior Member
I took the old one apart as it was frozen. Cleaned it up with steel wool, including the needle valve, and it's free but "sticky" and I can't adjust it accurately by turning the knob on the dash as there is too much free play when you turn the knob. I figured for less that $15 I'd just replace it. I made sure the choke rod was clean and straight, the shaft tube was clean, replaced all the grommets and that didn't help the free play. The car runs fine.......just can't adjust the GAV accurately.
Tom Endy
Well-known member
You need to make sure all the internal passageways are open. The attached article shows how to check them with a paper clip. If you can't get the brass GAV seat out you can still run a paper clip down through it and see if it can be seen at the bottom of the hole in the reservoir.
Another thought: If the jets have been manhandled and the comp jet is larger than the cap jet the GAV becomes useless.
Tom Endy
Another thought: If the jets have been manhandled and the comp jet is larger than the cap jet the GAV becomes useless.
Tom Endy
Tom Wesenberg
Well-known member
When I heat the area of the stuck seat, I use a propane torch and only heat it enough so it might be a very dull red in a perfectly dark room, then I run cold water over it.
This usually works with one heat, but once or twice I had to give it a second try to get the seat loose.
This usually works with one heat, but once or twice I had to give it a second try to get the seat loose.